An extruder is a machine in which material, usually a plastic material, is forced under pressure and often heat to flow through a chamber. The chamber has a feed port and a discharge port. Material flows out of the discharge port into post-extrusion apparatus adapted to further transform the material. Post-extrusion apparatus may include machines adapted to transform the material before it is forced to flow through contoured orifices, typically dies such blown or cast dies, or molding dies, or pipe dies, or pelletizing dies adapted to shape the material before the material solidifies into polymeric articles. Typical extruders include single screw and twin-screw extruders, and also injection molding machines.
Polymers are mixed with a wide range of additives in an extrusion composition to impart various properties to articles made therefrom. Exemplary properties of extruded articles include strength, color, clarity, and cost. Article properties depend in part on the selection of polymers, additives, processing aids, and also on the ability to mix such components into a suitable polymeric compound. As used herein, processing aids means components that are added to a composition for the purpose of improving processing. Exemplary components of extrusion compositions include solid polymers, molten polymers, inert and chemically active particles, powders, fibers, liquids and gases.
Components of an extrusion composition may be pre-blended before and while they are introduced into the extruder, mixed in the extruder, and further mixed in post-extrusion apparatus. Properly mixing particles such as nanoparticles and pigments is challenging in traditional extruders. As used herein, a nanoparticle is an individual particle measuring less than 1,000 nanometers (1 micron) in any one of its dimensions. Due to their size and structure, nanoparticles pose new extrusion mixing challenges. Pigments are also challenging due to their susceptibility to agglomeration and thermal degradation. It is particularly challenging to mix different pigments sufficiently to controllably create shades of colors, e.g., adding yellow and white to blue pigments to create shades of green polymers.
Extrusion mixing is challenging, at least in part, because conventional extrusion apparatus provide a limited number of material transformation controls. Their design implies a trade-off between the need to melt and convey a broad range of materials continuously and the specific mixing requirements of some compositions. As the characteristics of material compositions change, additional control variables may be needed to properly mix such compositions. New controls may be provided in post-extrusion apparatus, before melt solidification, suited to mix new material compositions or to achieve new levels of homogeneity.